1. Field of the Invention
This invention relates to novel hydrophobically associating terpolymers containing sulfonate functionality which are useful as aqueous fluid rheology or flow modifiers. These high molecular weight water soluble polymers contain both water soluble and water insoluble monomers. The water soluble monomers are acrylamide (AM) and a salt of an ethylenically unsaturated sulfonic acid (S) and the water insoluble monomer is a higher alkylacrylamide (R). These polymers will hereinafter be referred to as SRAM. The process for their preparation relies on solubilizing the water insoluble monomer into an aqueous micellar solution containing one or more surfactants and the water soluble monomers. The surfactants are specifically selected to enable homogeneous dispersion of the hydrophobic monomer(s) in the presence of anionic sulfonate containing monomers. Redox or free radical initiators are used to copolymerize both the water soluble and water insoluble monomers forming terpolymers of ethylenically unsaturated sulfonic acids, alkylacrylamides and acrylamide. Aqueous solutions of these hydrophobically associating polymers exhibit enhanced viscosification, reduced salt sensitivity and other desirable rheological properties found useful in a variety of applications.
2. Description of the Prior Art
Polyacrylamide (PAM) and partially hydrolyzed polyacrylamide (HPAM) are well known water soluble polymers used as flocculation aids for waste water treatment and dewatering sludge and for rheology control for secondary or tertiary oil recovery. Further examples of the properties and use of these polymers can be found in Handbook of Water Soluble Gums and Resins, R. L. Davidson, Ed., McGraw Hill, 1980, chapter 16 by H. Volk and R. E. Friedrich or in Water-Soluble Polymers, N. M. Bikales, Ed., Plenum Press, 1973, page 105, by D. C. Williams, J. H. Rogers and T. J. West. Polyacrylamides rely on a combination of high molecular weight and chain expansion due to repulsion of pendent ionic groups along the polymer chain to viscosify or thicken aqueous fluids. However, high molecular weight polymers mechanically degrade when subjected to large elongational or shear stresses, such as are found in pumps or during injection into reservoir rocks. This degradation results in permanent reduction in polymer molecular weight and, in turn, loss of viscosification efficiency. The presence of cations in aqueous solution, in particular divalent cations, shields the ionic charged groups on the polymer. This causes the polymer chains to collapse into a random coil-type configuration, losing significant viscosification efficiency. Thus, polymeric viscosifiers based on an alternative mechanism of viscosification, providing improved mechanical stability and salt tolerance relative to PAM and HPAM polymers would be highly desirable.
Water soluble copolymers of acrylamide and sulfonated monomers have been studied as aqueous fluid viscosifiers and flocculation agents. For example, C. L. McCormick and G. S. Chen, J. Polymer Science: Polymer Chemistry Ed., Volume 20, B17-B38 (1982) described the synthesis and characteristics of random copolymers of acrylamide and sulfonated monomers, such as sodium-2-sulfoethyl methacrylate or sodium-2-acrylamido-2-methylpropane sulfonate. In a recent paper on the dilute solution properties of these polymers, H. H. Neidlinger, G. S. Chen and C. L. McCormick, J. of Applied Polymer Science, Volume 29, 713-730 (1984) noted the high salt sensitivity of these polymers, particularly for copolymer compositions containing more than about 25 mole percent sulfonate monomer. U.S. Pat. No. 4,342,653 teaches the use of random copolymers of acrylamide and AMPS for the flocculation of suspended solids in such aqueous systems as phosphate slime, uranium acid leach residue, etc.
Processes for preparing polyacrylamides are well known in the art. Tanaka, et al., U.S. Pat. No. 4,154,910, teaches an aqueous solution method using the heat of polymerization to concentrate the product. Zimmerman, et al., U.S. Pat. No. 3,211,708, teaches an oil-in-water bead polymerization for polymerizing water soluble monomers. These techniques result in moderate molecular weight polymers exhibiting poor viscosification efficiency, particularly in saline solutions. Kolodny, U.S. Pat. No. 3,002,960, teaches a low temperature, redox initiated solution polymerization resulting in high molecular weight polyacrylamide. Another approach to high molecular weight water soluble polymers is described by J. W. Vanderhoff, et al., U.S. Pat. No. 3,284,393, where water soluble monomers are polymerized at high concentration in a water-in-oil emulsion. While some of these processes allow high molecular weight polymers to be prepared, the resulting polyacrylamide, PAM, and partially hydrolyzed polyacrylamide, HPAM, and copolymers of acrylamide and sulfonated monomer, SAM, systems provide only fair viscosification efficiency, poor mechanical stability and low salt tolerance.
One approach to overcoming the deficiencies in these polyacrylamide based systems is described by Bock, et al. in U.S. Pat. No. 4,520,182. Water soluble acrylamide copolymers containing a small amount of oil soluble or hydrophobic alkylacrylamide groups were found to impart efficient viscosification to aqueous fluids. Furthermore, these alkylacrylamideacrylamide copolymers (RAM) were nonionic and thus were relatively insensitive to the level of salt in the water. However, these polymers required concentrations above about 2,000 ppm to provide significant viscosification. Landoll, U.S. Pat. No. 4,304,902, describes copolymers of ethylene oxide with long chain epoxides which also required relatively large polymer concentration (approximately 1%) for thickening water and required surfactants for solubility due to irregularities in the polymerization. In a related case, U.S. Pat. No. 4,428,277, modified nonionic cellulose ether polymers are described. Although these polymers showed enhanced viscosification relative to polymers not containing hydrophobic groups, the viscosification efficiency was very low, requiring two to three weight percent polymer, to provide an enhancement. The use of surfactants to enable solubility and, in turn, viscosification by a water soluble polymer containing hydrophobic groups is described by Evani, European Patent No. 0 057 875. The hydrophobic groups claimed are attached to the polymer via an acrylate linkage which is known to have poor hydrolytic stability. In addition, the need for a surfactant to achieve solubility and thickening efficiency should make such a system very salt or temperature sensitive, as well as very sensitive to small changes in surfactant and polymer concentration. Emmons, et al., U.S. Pat. No. 4,395,524, teaches acrylamide copolymers as thickeners for aqueous systems. While these polymers possess hydrophobic groups, they are prepared using alcohol containing solvents which are known chain transfer agents. The resulting polymers have rather low molecular weights and, thus, relatively high polymer concentrations are required to achieve reasonable viscosification of water based fluids.
One of the objects of this invention is to overcome the deficiencies in the use of the water soluble polymers of the prior art for thickening aqueous fluids. A new class of water soluble polymer will be described which impart enhanced viscosification to aqueous fluids, improved mechanical stability and better salt tolerance. These new polymers contain a nonionic water soluble monomer, such as acrylamide, an anionically charged water soluble ethylenically unsaturated sulfonated monomer, such as an alkali metal salt of a vinyl alkylsulfonic acid, and a water insoluble or hydrophobic monomer, such as an alkylacrylamide, with a chain length of six carbons or greater. When these polymers are placed in an aqueous solvent, the hydrophobic groups aggregate or associate in a manner similar to a surfactant. This hydrophobic association between polymer chains in solution results in an increase in the hydrodynamic size of the molecule, which, in turn, causes an increase in viscosity. We have found that the presence of ionic groups, such as sodium 2-acrylamide-2-methylpropane sulfonate, AMPS.RTM., cause an expansion of the polymer in solution, an improvement in polymer solubility and a favorable interaction with the hydrophobic groups. Thus, polymers containing both ionic sulfonate groups and hydrophobic groups provide a significant improvement in viscosification efficiency of water based systems.
Synthesis of the hydrophobically associating polymers of the instant invention presents difficulties. In order for polymerization to be effected the monomers must obviously come into close proximity to one another. The incompatibility of the oil soluble and water soluble monomers prevents an effective concentration of one or the other of these monomeric species from being achieved at the locus of polymerization of the other comonomer. Thus, a method for preparing the novel sulfonate containing hydrophobically associating polymers of this invention is needed. A variety of processes based upon prior art could conceivably achieve this, but have serious deficiencies. For example, simply dispersing the water insoluble monomer as fine particles in the aqueous medium containing dissolved water soluble monomers would result in low incorporation of the water insoluble monomer and would lead to a heterogeneous product of particles dispersed in a predominantly water soluble polymer. The resulting polymer could not be used to impart efficient and uniform thickening to water based fluids.
Techniques for polymerizing water soluble polymers, such as those taught in U.S. Pat. Nos. 4,154,190, 3,211,708, 3,002,906 and 3,284,393, cannot be used to prepare the compositions of this invention. This art does not teach the formation of a sufficiently fine dispersion of the water and oil soluble monomers to enable uniform reaction and efficient aqueous viscosifiers to be prepared. The use of mutual solvents or solvent mixtures to dissolve the water and oil soluble monomers, as taught by Lenke, et al., U.S. Pat. No. 4,151,333, and Barua, et al., U.S. Pat. No. 4,098,987, have some serious limitations. Although this approach undoubtedly allows the incompatible monomers to come into close proximity to one another, since the dispersion is on a molecular scale, often the resulting copolymer is insoluble in the same solvent as shown in U.S. Pat. No. 4,151,333. This leads to precipitation of the copolymer before it has achieved sufficient molecular weight to provide efficient aqueous viscosification. The use of water miscible solvents, such as alcohols, ether and acetone, either alone or with water, as taught in U.S. Pat. No. 4,098,987, results in extremely low molecular weight (e.g., 10,000) polymers due to the high chain transfer characteristics of these solvents. Thus, polymers produced by these teachings are rather ineffective viscosifiers for aqueous fluids.
Two techniques have been found most useful for preparing hydrophobically associating copolymers of acrylamide and alkylacrylamides. The first method was based on the use of a water continuous microemulsion to disperse the oil soluble monomer in a solution of the water soluble monomers. Details of the procedures and techniques are taught by Turner, et al., U.S. Pat. No. 4,521,580. A second method for preparing copolymers of acrylamide and alkylacrylamide was based on dispersing the oil soluble monomers using an aqueous micellar solution of the water soluble monomers. Suitable surfactants and the details of the polymerization are taught by Turner, et al. in U.S. Pat. No. 4,528,348. While either the microemulsion or micellar polymerization techniques can be used to prepare hydrophobically associating polymers containing a variety of water soluble nonionic monomers, a problem arises when the monomers have a strong interaction with the surfactants used in the polymerization. In particular, strong ionic interactions or complexes can be formed between anionic water soluble monomers, such as ethylenically unsaturated alkyl sulfonates, and cationic surfactants, such as quaternary alkyl amines. A copending application, U.S. Pat. No. 4,730,028, teaches a means of overcoming these deficiencies to enable preparation of the novel hydrophobically associating terpolymersof this invention.